Overpotential methanol oxidation reaction

At high anodic overpotentials, methanol oxidation reaction exhibits strongly non-Tafel behavior owing to finite and potential-independent rate of methanol adsorption on catalyst surface [244]. The equations of Section 8.2.3 can be modified to take into account the non-Tafel kinetics of methanol oxidation. The results reveal an interesting regime of the anode catalyst layer operation featuring a variable thickness of the current-generating domain [245]. The experimental verification of this effect, however, has not yet been performed. 

There is another fuel cell working under the ambient condition, that is, direct methanol fuel cells (DMFCs). Difference in the PEFCs and DMFCs is their anode fuels (the cathode fuel is oxygen in both cases). In the DMFCs, methanol (CH3OH) is supplied to the anode instead of the hydrogen for the PEFCs and this difference is crucial for their ceU performances. Although the Pt is known to be an active catalyst for both HOR and methanol oxidation reaction (MOR), kinetics of the MOR is much slower than that of the HOR and ORR on the Pt catalyst, which increases anode overpotential and gives an inferior cell performance in the DMFCs as demonstrated in Fig. 1. Therefore, an important research topic is... 

Rate determining step (cont.) electrocatalysis and, 1276 methanol oxidation, 1270 in multistep reactions, 1180 overpotential and, 1175 places where it can occur, 1260 pseudo-equilibrium, 1260 quasi equilibrium and, 1176 reaction mechanism and, 1260 steady state and, 1176 surface chemical reactions and, 1261 Real impedance, 1128, 1135 Reciprocal relation, the, 1250 Recombination reaction, 1168 Receiver states, 1494 Reddy, 1163... 

The Cat, or its product of electrode oxidation or reduction Cat, is immobilized at the electrode surface and decreases the overpotential for oxidation or reduction of the S, without being involved in the chemical redox reaction with the S. Typical example is the catalytic effect of underpotential deposited layer of lead on a platinum electrode, on anodic oxidation of methanol [v]. 

In this section we will discuss the role of surface modification to enhance electrocatalytic oxidation of methanol, one of the interesting components for fuel cell technology. Perhaps the most successful promoter of methanol electrooxidation is ruthenium. Pt/Ru catalysts appear to exhibit classical bifunctional behavior, whereas the Pt atoms dissociate methanol and the ruthenium atoms adsorb oxygen-containing species. Both platinmn and ruthenimn atoms are necessary for eomplete oxidation to occur at a significant rate. The bifunctional mechanism can account for a decrease in poisoning from methanol, as observed for Pt/Ru alloys. Indeed, CO oxidation has been attributed to a bifimctional mechanism that reduces the overpotential of this reaction by 0.1 V on the Pt/Ru surface. 

On the other hand, the Pd/MWCNT electrode is electrochemically stable only for the oxidation of ethanol as the oxidation reactions of methanol and glycerol are featured by a fast increase of the overpotential (Fig. 13). 

Due to the slow oxidation kinetics, methanol will hardly be oxidized with a reasonable reaction rate at potentials lower than about 0.3 V. This can be seen in Figure 7.1, which shows the methanol oxidation process. Once the voltage was higher than 0.3 V, the oxidation current increased significantly. When the current density went beyond about 0.14 A cm, a steep overpotential increase started due to the mass transport resistance for this particular experiment. For current densities between 0.03 and 0.14 A cm", the overpotential versus the current density was linear, primarily due to the iR loss. 

In fuel cells working with a liquid fuel, usually an alcohol such as methanol (a direct methanol fuel cell - DMFC), ethanol (a direct ethanol fuel cell - DEFC), glycerol (a direct glycerol fuel cell - DGEC), etc., in addition to the necessity to activate the ORR at the cathode, the alcohol oxidation reaction at the anode also involves a high overpotential. This high overpotential is mainly due to the formation, after dissociative adsorption of the alcohol at the catalyst surface, of poisoning species which block the catalytic surface the main one adsorbed is carbon monoxide. ... 

Figure 12.10 shows the CVs for methanol oxidation on a Pt/Ru catalyzed electrode at various temperatures. It can be seen that there is a significant increase in the current density with increasing temperature. From the data in this figure, the kinetic parameters of methanol oxidation can be estimated. For an electrochemical reaction controlled purely by electron transfer kinetics, if the reaction overpotential is large enough (>60mV), the Butler-Volmer equation can be simplified to the form of a Tafel equation, which is similar to Eqn (12.6) ... 

Reaction Mechanism In general, the more complex a reaction mechanism, the greater the overpotential required to break the chemical bonds and generate current. For example, the HOR is less complex and involves fewer intermediate steps than the methanol electrooxidation, so that for the same current the overpotential for methanol oxidation is greater than for hydrogen oxidation. There are steric (geometric) and other factors involved as well. 

Poisoning of platinum fuel cell catalysts by CO is undoubtedly one of the most severe problems in fuel cell anode catalysis. As shown in Fig. 6.1, CO is a strongly bonded intermediate in methanol (and ethanol) oxidation. It is also a side product in the reformation of hydrocarbons to hydrogen and carbon dioxide, and as such blocks platinum sites for hydrogen oxidation. Not surprisingly, CO electrooxidation is one of the most intensively smdied electrocatalytic reactions, and there is a continued search for CO-tolerant anode materials that are able to either bind CO weakly but still oxidize hydrogen, or that oxidize CO at significantly reduced overpotential. 

Thus, the reaction is poisoned by the formation of Pt-(CO)ads after complete methanol dehydrogenation. There have been intensive searches for other active materials, which can provide oxygen in its active form from water at low overpotentials, to increase the oxidation rate of chemisorbed CO. Pt-Ru alloys are reported as having excellent promotional effects, " and PtSn,[ l PtMo,t ° PtRuCo, and PtRuIrOs have also been studied for the MOR reaction of DMFC. 

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